We wish to develop a systematic procedure allowing the identification of lesions occurring in DNA to the extent of 100 parts per million or less. In the first stage of the investigation, we are concentrating upon one particular type of damage: covalent cross-links between the two chains of DNA. A number of reagents have been described which cross-link DNA, but the nature of the cross-link is unknown in most cases. We have recently devised new procedures applicable to the isolation of cross-linked nucleosides from DNA (S. Dubelman and R. Shapiro, Nucleic Acids Res., in press (1977)). The application of these techniques to DNA cross-linked by nitrous acid led to the identification of 2 cross-linked nucleosides (R. Shapiro, et. al., J. Amer. Chem. Soc. 99, 302(1977)). We are currently studying the structures of nucleosides cross-linked by formaldehyde, using products isolated from DNA and RNA, as well as those produced by direct reaction of nucleosides with formaldehyde. In the future we shall be examining the cross-linking of DNA by butadiene diepoxide. A fraction of our efforts is being devoted to devising new techniques for the identification of modified nucleosides at the microgram level. Microchemical tests and extensions of the use of ultraviolet and mass spectroscopy will be applied to this purpose. BIBLIOGRAPHIC REFERENCES: R. Shapiro, S. Dubelman, A. Feinberg, P.F. Crane and J. McCloskey, "Isolation and Identification of Cross-Linked Nucleosides from Nitrous Acid Treated Deoxyribonucleic Acid," J. Amer. Chem. Soc. 99, 302 (1977). H. Szejnwald Brown and R. Shapiro, "Preparation and Enzymatic Hydrolysis of Dinucleoside Monophosphates and DNA Modified with Aromatic Residues," Biochim. Biophys. Acta 475, 241 (1977).